1. Field of the Invention
This invention relates to diaphragms for speakers or acoustic transducers and more particularly, to integrally molded diaphragm-edge articles which are adapted for use in acoustic output apparatus. The invention also relates to methods for fabricating the diaphragm-edge integral moldings and to acoustic transducers comprising the same.
2. Description of the Prior Art
As is well known in audio and allied industries, digitalization of reproduction music sources has advanced materially. This makes a great demand for speakers, which are higher in sound quality than conventional counterparts, for use in acoustic output apparatus.
One of physical properties required for the diaphragm of speakers is stiffness of diaphragm material. The improvement of the stiffness contributes to suppressing partial vibrations such as surface resonance and reducing distortion rates, ensuring reproduction of higher frequency components. The physical characteristics required for materials for the edge portion include flexibility, by which distortions with the diaphragm are suppressed, enabling reproduction of lower frequency components. In order to satisfy both requirements, usual practice is to use a structure which makes use of different types of materials for both diaphragm and edge or surround portions. For instance, with microspeakers having a diameter of not larger than 40 mm, it is usual from the standpoint of their structural arrangement and fabrication cost to integrally mold diaphragm and edge portions from a single material such as a film of polyethylene terephthalate resin (PET) or polycarbonate (PC). However, the integral molding from such a single material is disadvantageous in that if the stiffness of the diaphragm is increased in order to improve a high-band threshold frequency, f.sub.h the edge increases in stiffness, so that a minimum resonance frequency, f.sub.o, is simultaneously shifted toward a higher frequency band. On the contrary, when the stiffness of the edge is decreased in order to decrease the value of f.sub.o the stiffness of the diaphragm is lowered with f.sub.h being shifted toward a lower frequency band. More particularly, it is not possible to satisfy the requirements for both diaphragm and edge, which are contrary to each other, in order to realize broad band frequency characteristics, thus resulting in narrow band frequency characteristics. In addition, limitation is placed on the inherent movements of the edge and the diaphragm of speaker as will be required by application of reproduction signals, generating an excessive distortion, Hence, it has been difficult to stably reproduce HiFi audio sound from compact disks and PCM sound sources in a frequency band of from 20 to 20,000 Hz.
Moreover, with speakers having a larger diameter and making use of different types of materials for the diaphragm and edge, respectively, the integral molding of diaphragm-edge has not been generally employed because of the difficulty in establishing molding or shaping conditions of different types of materials and the complication of molding apparatus. At present, diaphragm and edge pieces are separately fabricated, after which both pieces are bonded together through a bonding step. This presents many problems such as a problem of separation between the once bonded pieces and a problem on bonding agents or adhesives from which volatile solvents undesirably evaporate.